With recent developments in silver halide photographic light-sensitive materials, particularly light-sensitive materials for photography, light-sensitive materials having high sensitivity, as typically exemplified by films of ISO 1000, have become strongly desired. Various methods to increase sensitivity have hitherto been attempted by, for example, increasing the size of the silver halide grains increasing activity of couplers, accelerating development, and the like. However, any further increase in sensitivity by using large-sized silver halide grains cannot be expected, seeing that achievement in increasing sensitivity by using large-sized silver halide grains seems to have reached the limitation as reported in G. C. Farnell and J. B. Chanter, Journal of Photographic Science, Vol. 9, 75 (1961). Furthermore, increasing the grain size is accompanied with various disadvantages, such as deterioration in improved graininess or preservability of an emulsion layer in projection to increasing thickness. Further, use of highly active couplers or acceleration of development is not only accompanied by significant deterioration in graininess, but such is also insufficient in contributing to increases in sensitivity. Furthermore, addition of various development accelerators, such as hydrazine compounds, to an emulsion layer or a developing solution, typically in achromatic light-sensitive materials, has been studied for the purpose of accelerating development. However, conventional development accelerators are not fully satisfactory due to a tendency to increase fog and/or deteriorate graininess.
Hence, couplers capable of imagewise releasing a development accelerator or a fogging agent have been proposed. For example, U.S. Pat. Nos. 3,214,377 and 3,253,924 and Japanese Patent Application (OPI) No. 17437/76 (the term "OPI" as used herein means an "unexamined published application") disclose couplers capable of releasing thiocyanate ions which accelerate a physical phenomenon of dissolution. Further, Japanese Patent Application (OPI) No. 138636/82 discloses couplers capable of releasing hydroquinone or aminophenol developing agents.
However, the groups released from these couplers exhibit insufficient development accelerating activity or fogging activity so that use of these couplers brings about only a little effect. In order to overcome this problem, couplers capable of releasing acylhydrazines as described in Japanese Patent Application (OPI) No. 150845/72 and couplers capable of releasing thiocarbonyl compounds as described in Japanese Patent Application (OPI) No. 161515/82 have been proposed to realize increase of sensitivity of means of couplers.
Nevertheless, since the released groups from these couplers are highly diffusible, when the couplers are incorporated in a layer sensitive to a certain color, the released compound is also diffused into other layers sensitive to a different color to thereby exhibit development accelerating activity or fogging activity therein, thus causing color mixing. Therefore, use of these couplers turns out to be disadvantageous in that color reproducibility is extremely deteriorated.
More specifically, in light-sensitive materials containing a compound capable of imagewise releasing of a fogging agent or a development accelerator or a precursor thereof (hereinafter referred to as "FR compound", and the compound capable of releasing FR compounds is hereinafter referred to as "FR-releasing compound") in accordance with the amount of developed silver, the FR compound released upon development donates an electron to the silver halide in the light-sensitive material or forms silver sulfide to increase centers of development, and thus accelerates development. As a result, favorable effects, such as an increase in sensitivity, increase in contrast, acceleration of development, and the like can be attained. On the other hand, a part of the FR compound imagewise released in a layer having a certain color sensitivity diffuses into other layers having different color sensitivity and accelerates development imagewise of said color sensitivity over unnecessarily enlarged portions of the light-sensitive material. This results in significant color mixing and extreme deterioration of color reproducibility
Various other additives have also been studied in to increase sensitivity, but did not succeed due to considerable side effects.